The invention relates to the field of ellipsometry, and more particularly to ellipsometry operating in the infrared.
It has a general application in any field in which ellipsometry is used, and more particularly in microelectronics, in the optical characterisation of a sample, in the optical control of a surface treatment, or in the study of the growth of thin layers, for example of semiconductor materials, and of their interfaces. It also has an application in the cleaning, polishing and preparation of surfaces, in particular.
Ellipsometric measurements can be carried out at a fixed wavelength (monochromatic ellipsometry) or at several wavelengths (spectroscopic ellipsometry).
Depending upon the wavelength range of the source: ultraviolet, visible, near infrared, infrared, etc., it is possible to attain different properties of the layers, of the materials or to explore different materials.
In practice, the infrared is generally better adapted than the visible to attain the voluminal properties of the layers and of the materials.
In general, an ellipsometer operating in the infrared comprises:
a source of light radiation supplying at least one infrared beam;
a sample holder intended to carry a sample of a given thickness and comprising a front face and a rear face;
a detector;
a first optical system mounted between the source and the sample holder, and comprising a polariser and a focusing device in order to illuminate the sample placed on the sample holder under oblique incidence by a beam of polarised light; and
a second optical system mounted between the sample holder and the detector and comprising a focusing device and an analyser to collect the light reflected by the sample.
Because of the very strict requirements in the manufacture of semiconductors there is a need for an ellipsometer having a high spatial resolution and the best possible accuracy of measurement.
With a transparent or semi-transparent sample of a given thickness, such as silicon, the rear face of the sample can disrupt the ellipsometric measurements by reflecting the stray radiation which contaminates the detection and the processing of the useful signal.
This contamination is difficult to deal with because the reflection coefficient of the rear face of a silicon substrate is not always known. The absorption coefficient k of the substrate is also not always known. Interference phenomena can also occur with this rear face. Likewise, phenomena of diffusion and/or of diffraction n infrared can also occur on this rear face. Moreover the rear face may not be parallel to the front face, which can give rise to additional and unnecessary calculations because only the measurement of the front face is pertinent for the user.
Known solutions exist for elimination of the harmful effects brought about by the rear face of the sample, particularly the use of mechanical means such as means for dulling the rear face which renders the specular reflections emitted by the rear face negligible.
When the sample has a relatively great thickness, it is possible to separate the radiation emitted by the front face from the radiation emitted by the rear face. However, such a solution can only be achieved for samples of a great thickness, which limits its application.
Another known solution consists of taking absorbent samples (that is to say non-transparent samples, such as highly doped silicon), but this also limits the application of such a solution.
The object of the present invention is to remedy these drawbacks and to propose a high spatial resolution ellipsometer which operates in the infrared in which stray reflections due to the rear face of the sample are eliminated.
The present invention relates to an ellipsometer device of the type comprising:
a source of light radiation supplying at least one infrared beam;
a sample holder intended to carry a sample of a given thickness and comprising a front face and a rear face;
a detector;
a first optical system mounted between the source and the sample holder, and comprising a polariser and a focusing device in order to illuminate the sample placed on the sample holder under oblique incidence by a beam of polarised light; and
a second optical system mounted between the sample holder and the detector and comprising a focusing device and an analyser to collect the light reflected by the sample.
According to a general definition of the invention, the ellipsometer device further comprises a blocking device mounted on the reflection path in the focal plane of the focusing device of the second optical system and capable of blocking the stray radiation emitted by the rear face of the sample and allowing the useful radiation emitted by the front face of the sample to pass through towards the detector, which makes it possible to obtain a separating power with regard to the front and rear faces of the sample.
For example, the blocking device is of the type constituted by a slot with adjustable dimensions, cutter with adjustable edge, or the like.
According to a first preferred embodiment of the invention, the ellipsometer device according to the invention further comprises a widening device mounted on the illumination path and capable of widening the illuminating beam on the focusing device of the first system and of widening the reflecting beam on the focusing device of the second optical system.
For example, the widening device is of the type constituted by a slot with adjustable dimensions, cutter with adjustable edge, divergent lens, or the like.
The numerical aperture of the focusing device of the first optical system is preferably chosen so as to obtain an illuminating beam of small size on the sample. For example the size of the illuminating beam on the sample is less than 40 micronsxc3x9740 microns in the case of a light source of the laser type.
For example, the focusing devices of the first optical system as well as of the second optical system comprise at least one optical element belonging to the group formed by concave mirrors (for example elliptical, parabolic, spherical, etc.).
The numerical aperture of the focusing device of the second optical system is chosen to separate the beams reflected by the front and rear faces of the sample.
According to a second preferred embodiment of the invention the ellipsometer device according to the invention further comprises a selector device for angles of incidence, mounted on the reflection path downstream of the blocking device according to the direction of propagation of the light and capable of selecting, for the measurements by the detector, only the radiation reflected by the sample under oblique incidence within a predetermined range of angles of incidence.
The selector device is preferably of the type constituted by a slot with adjustable dimensions, cutter with adjustable edge, or the like.
In practice, the light source is of the laser type, operating at terahertz frequencies, or silicon carbide source, filament, plasma, or the like.
The polariser of the first optical system is preferably of the type having a grid, with or without a rotating compensator, assembly with several polarisers with grids, or the like.
Likewise, the analyser of the second optical system is of the type having a grid, with or without a rotating compensator, assembly with two polarisers with grids, or the like.
For example, the detector is of the type constituted by a mercury-cadmium and/or tellurium cell, liquid nitrogen or the like.
In practice, the sample holder is of the type constituted by a table which is movable in XYZ and/or in rotation, a suspended sample holder or the like.